1. Field of the Invention
The invention concerns a fitting for sealed termination of a length of corrugated tubing, especially corrugated stainless steel tubing, for example as used for gaseous fuel lines.
The fitting includes a threaded fitting body and nut, cooperating with a retainer having one or more split rings that grip the corrugations adjacent to an end of the tubing. The tubing preferably is cut near a minimum diameter between corrugations and the retainer is placed axially behind the endmost corrugation. The nut and fitting body are placed on axially opposite sides of the retainer near the cut end. Tightening the nut on the fitting body forces the cut end of the tube axially against the fitting body, which has a dual sealing structure for providing a thin circular metal/metal sealing junction and an adjacent metal/gasket sealing junction.
More particularly, an inwardly tapering conical surface of the fitting body has a sharply formed outer edge. This sharp edge is surrounded by an annular space containing a gasket. During tightening, the cut end of the tubing deforms radially inwardly at the conical surface, causing the adjacent corrugation to fold and flatten over the sharp edge. A circular metal/metal junction with the tubing occurs at the nip between the retainer and the sharp edge surrounding the conical surface. The edge is placed to fall between the maximum and minimum radii of the corrugation of the tubing. The radially outer part of the collapsed corrugation forms a torus or bead that compresses the gasket, thereby also forming a metal/gasket seal.
2. Prior Art
Flexible corrugated tubing, especially corrugated stainless steel with optional plastic cladding, is an advantageous choice for natural gas supply lines and other applications that need to be gas- and/or water-tight as well as durably protected. The flexibility of corrugated tubing allows accommodates fixed and movable variations in the orientation and spacing of the connections of tube ends. This sort of tubing also is durable and resistant to damage from punctures and crushing. The flexibility minimize metal fatigue cracking due to repeated flexing. The corrugations can be engaged in the terminating fittings, providing strong mechanical connections that can bear substantial tension without being pulled apart.
However, couplings made at the ends of lengths of corrugated tubing are somewhat of a challenge. The corrugated tubing needs to be sealed and mechanically attached to associated terminal fittings to as to form a leak resistant flow path. The element that mates with the corrugated tube termination fitting could be a rigid supply pipe having a pipe thread fitting, for example, or a structure of an appliance, or perhaps an intermediate device such as a tee or a diameter changing nipple, valve, manifold, filter, etc.
The mechanical connection as well as the seal between the corrugated tubing and the terminal device or fitting should remain hermetically tight and mechanically load bearing over the life of the connection, which often equates to the life of the associated appliance or connection line. The tubing may be used to carry flammable gas to an appliance, and should survive adverse conditions without leakage. For example, the seal should remain gas-tight even in high temperature conditions as one might expect in a fire.
Various terminal fittings for corrugated tubing are known and are intended to provide a good mechanical connection and hermitic seal. The known fittings have a range of complexity. Some aspects that distinguish fitting structures over one another, in addition to mechanical attachment and sealing effectiveness, include the expense, the number and complexity of the parts, the steps required to assemble the fitting on a tube end, whether the parts are consumed or re-usable, etc.
Establishing a seal may involve axial or radial pressure exerted between the tubing and the fitting on an intervening gasket or O-ring. However, such pressure may be achieved in various ways operating axially or radially or both. Achieving secure mechanical contact against tension generally involves providing retaining structures such as split retainer rings that have one or more annular ridges extending radially inwardly into the valleys between corrugations so as to grip the tube against axial displacement. Advantageously, at least two parts are brought together during assembly of the fitting on the end of the tube, and the two parts can be arranged to engage the gripping structure for pushing the corrugated tubing against some sort of structure that is intended to provide a connection. The two parts respectively engage with the gripping structure and the structure against which the tubing is to be urged.
Examples of terminal fittings as described, generally for annularly corrugated tubing, are disclosed in U.S. Pat. No. 4,630,850—Saka; U.S. Pat. No. 5,441,312—Fujiyoshi et al.; U.S. Pat. No. 5,799,989—Albino; U.S. Pat. No. 6,019,399—Sweeney; U.S. Pat. No. 6,036,237—Sweeney: U.S. Pat. No. 6,173,995—Mau; and U.S. Pat. No. 6,276,728—Treichel. There are two general approaches to sealing represented. One technique is to provide a resiliently compressible gasket, and to arrange for a metal part to bear against the gasket. Another technique is to provide a metal-to-metal clamping mechanism involving the corrugated tubing, typically designed to crimp and flatten one or more corrugations between vise-like abutting surfaces that are brought axially together over a corrugation that is flattened.
It may be possible axially to seal the cut end of a corrugated tube by arranging for the cut end to bear against an annular abutting surface, possibly having a compressible gasket. However, the cut end of the tubing may not be cut exactly on a plane perpendicular to the axis of the tube. Different sorts of tools may be used to make the cut, which affect the nature of the cut (e.g., a hacksaw versus a pipe cutter). The cut edge may have irregularities from the cutting tool. Unless special provisions are available, the cut edge may occur at any phase position along the periodic corrugations, between the maximum and minimum diameter. As a result, the cut edge may be directed more or less axially versus radially for any give cut. These variations complicate the possibility of a direct endwise seal between the cut end and an abutting surface arranged substantially in a plane normal to the axis of the tube.
To reduce the possibility that unevenness at the cut end could result in a gap, it is conventional to clamp part of the endmost corrugation(s) between metal surfaces that are brought together when tightening the fitting. These surfaces flatten or reform one or more of the endmost corrugations so as to provide a flattened radial flange that is clamped between annular gripping surfaces on the fitting body and the retaining ring. sealing surfaces of greater width than the material thickness at a cut end. Flat annular surfaces in a plane normal to the longitudinal axis of the tubing are one possibility and can be formed using a split ring or other grasping structure the extends radially inwardly into a corrugation and is caused by some sort of collar to axially compress and clamp the adjacent corrugation. It is possible to use a radially flat annular clamping surface or conical clamping surfaces.
It is conventional to connect corrugated tubing to a fitting by flattening one or more corrugations of the tubing into flanges that are compressed between respective surfaces of a gripping or corrugation-engaging retainer and a complementary flat annular surface on the fitting body. Forming a flat flange reduces the effect of the cut end not being precisely formed. However a wide flat area of contact between a flange and a corresponding flat annulus may not provide a highly effective metal/metal seal, or a distinct edge to bear against a compressible gasket material.
To some extent, providing metal-to-metal sealing engagement is choice that is mutually exclusive with providing a metal-to-compressed gasket sealing. Each has different advantages and disadvantages. Metal-to-metal seals are strong but comprise material with comparatively little ability to recover their shape after deformation. Deformation that occurs when metal sealing structures are brought into contact is more or less permanent. In comparison, compressible materials such as resilient gasket material, can be deformed to complement an irregular shape. If a compressible material is later moved (for example when a fitting is taken apart and then reconnected), the compressible material can recover its original shape to an extent, and be compressed again under slightly different conditions to attain a new seal. Metal/metal seals are less likely to produce a new hermetic seal if disturbed.
It is possible to provide spring-like metal that has resilience, but it is generally not compressible in a way that facilitates sealing along a distinct sealing surface or edge. The metal material has advantages of structural strength, and disadvantages as to compliance for sealing. A more malleable metal is possible to conform under pressure, but is permanently deformed and lacks compliance under changing conditions.
It would be advantageous to maximize the benefits of metal/metal sealing strength while also providing sealing along a distinct edge as opposed to a wide surface. It would also be advantageous to make a seal compliant and re-usable even though the structures involved make substantial use of metal contact. It would further be advantageous to provide good metal-to-metal sealing in a structure that also is compliant to the extent that the fitting is insensitive to the precision or lack of precision along its cut end, is compliant and re-usable, and is unlikely in the long term to develop a leak.